Developments in zeolite catalysts and hydrocarbon conversion processes have created interest in utilizing light aliphatic feedstocks for producing C.sub.5 + gasoline, diesel fuel, etc. In addition to basic chemical reactions promoted by medium-pore zeolite catalysts, a number of discoveries have contributed to the development of new industrial processes. These are safe, environmentally acceptable processes for utilizing feedstocks that contain olefins. Conversions of C.sub.2 -C.sub.4 alkenes and alkanes to produce aromatics-rich liquid hydrocarbon products were found by Cattanach (U.S. Pat. No. 3,760,024) and Yan et al (U.S. Pat. No. 3,845,150) to be effective processes using the zeolite catalysts. In U.S. Pat. Nos. 3,960,978 and 4,021,502, Plank, Rosinski and Givens disclose conversion of C.sub.2 -C.sub.5 olefins, alone or in admixture with paraffinic components, into higher hydrocarbons over crystalline zeolites having controlled acidity. Garwood et al have also contributed to the understanding of catalytic olefin upgrading techniques and improved processes as in U.S. Pat. Nos. 4,150,062, 4,211,640 and 4,227,992. The above-identified disclosures are incorporated herein by reference.
Catalytic aromatization of light paraffinic streams, e.g. C.sub.2 -C.sub.4 paraffins, commonly referred to as LPG, is strongly endothermic and typically carried out at temperatures between 540.degree. and 820.degree. C. (1000.degree. and 1500.degree. F.). While the incorporation of hydrogenation/dehydrogenation metals including gallium, platinum, indium, tin and mixtures thereof in zeolite catalysts may reduce the operating temperature to the range of about 400.degree. to 600.degree. C. (750.degree. to 1100.degree. F.), the problem of transferring sufficient heat to a catalytic reaction zone to carry out the paraffin upgrading reaction remains as an obstacle to commercialization of the process.
Methods of supplying heat to the endothermic reaction zone include indirect heat exchange, e.g. a multi-bed reactor with inter-bed heating. Direct heat exchange techniques include oxidative dehydrogenation of a portion of the feedstream. Unfortunately, however, oxidative dehydrogenation is accompanied by a loss of a valuable by-product, hydrogen. Further, the incremental costs associated with maintaining a controlled supply of a suitable oxygen source, e.g. NO.sub.x, CO.sub.2 or SO.sub.3, makes commercialization of such schemes impractical. Clearly, for the coupling of an exothermic reaction with the endothermic paraffin upgrading process to be economically beneficial, the exothermic reaction must not defeat the economic viability of the primary conversion reaction.
Processes for converting lower oxygenates such as methanol and dimethyl ether to hydrocarbons are known and have become of great interest in recent times because they offer an attractive way of producing liquid hydrocarbon fuels, especially gasoline, from sources which are not of liquid petroliferous origin. In particular, they provide a way by which methanol can be converted to gasoline boiling range products in good yields. The methanol, in turn, may be readily obtained from coal by gasification, to synthesis gas and conversion of the synthesis gas to methanol by well-established industrial processes. As an alternative, the methanol may be obtained from natural gas by other conventional processes.
The conversion of methanol and other lower aliphatic oxygenates to hydrocarbon products may take place in a fixed bed process as described in U.S. Pat. Nos. 3,998,899; 3,931,349 (Kuo) and 4,035,430. In the fixed bed process, the methanol is usually first subjected to a dehydrating step, using a catalyst such as gamma-alumina, to form an equilibrium mixture of methanol, dimethyl ether (DME) and water. This mixture is then passed at elevated temperature and pressure over a catalyst such as ZSM-5 zeolite for conversion to the hydrocarbon products which are mainly in the range of light gas to gasoline. Water may be removed from the methanol dehydration products prior to further conversion to hydrocarbons and the methanol can be recycled to the dehydration step, as described in U.S. Pat. No. 4,035,430. Removal of the water is desirable because the catalyst may tend to become deactivated by the presence of excess water vapor at the reaction temperatures employed; but this step is not essential.
Thermal balance is a major problem in the operation of an adiabatic process. Process development involving exothermic reactions, e.g. conversion of methanol to hydrocarbons over a zeolite-containing catalyst, clearly demonstrates the significant impact of the problem of dissipating excess thermal energy as well as the costs for heat removal equipment.
The conversion of the oxygenated feed stream (methanol, DME) to the hydrocarbons is a strongly exothermic reaction liberating approximately 1480 kJ. (1400 Btu) of heat per kilogram of methanol. In an uncontrolled adiabatic reactor this would result in a temperature rise which would lead to extremely fast catalyst aging rates or even to damage to the catalyst. Furthermore, the high temperatures which might occur could cause undesirable products to be produced or the product distribution could be unfavorably changed. It is therefore necessary that some method should be provided to maintain the catalyst bed within desired temperature limits by dissipating the heat of the reaction.
One method is to employ a light gas portion of the hydrocarbon products as recycle, as desccibed in U.S. Pat. No. 3,931,349 (Kuo). Typically, cooled light hydrocarbon gas, rich in methane, ethane, etc., is separated from the gasoline and LPG products, re-compressed and reheated before being mixed with the reactant feedstream entering the bed of conversion catalyst. Although effective in controlling bed temperature, the expense of cooling the recycle gas, compressing it and re-heating it add to the cost of the conversion, indicating that a reduction in recycle ratio would be economically desirable. The recycle ratio can indeed be decreased but only with certain disadvantages. Not only will the temperature rise across the catalyst bed be greater, thereby increasing the aging rate of the catalyst but, in addition, the reactor must be operated at a lower and generally less favorable temperature; the outlet temperatures must be lowered in order to protect the catalyst from the increased partial pressure of the water which is consequent upon the lower partial pressure of the recycle gas and the inlet temperature must be lowered even further in order to compensate for the greater temperature rise across the catalyst bed. This is generally undesirable because the octane number of the gasoline product is related to reactor temperature with the higher octane products being produced at the higher temperatures. There is also a minimum reactor inlet temperature that must be maintained for the conversion to proceed and consequently, there is a limit on the extent to which the recycle ratio can be reduced.
A similar proposal is set out in U.S. Pat. No. 4,404,414. The process described in this patent employs a number of fixed bed reaction zones in which oxygenated feedstock is converted to hydrocarbon products by means of contact with a conversion catalyst. The temperature in the reactors is maintained at the desired value by the use of a diluent which is passed through the reactors in sequence before it is completely cooled and separated from the conversion products. The diluent in this case is light hydrocarbon gases which have been separated from the liquid hydrocarbon products and water. Once again, the expense of cooling the recycle gas, compressing it and re-heating it add to the cost of the conversion.
A somewhat similar challenge involves supplying the required heat for processes involving highly endothermic reactions.
U.S. Pat. No. 3,136,713 to Miale et al teaches a method for heating a reaction zone by selectively burning a portion of a combustible feedstream in a reaction zone. Heat is directly transferred from the exothermic oxidation reaction to supply the endothermic heat for the desired conversion reaction.
Heat balanced reactions are also taught in U.S. Pat. Nos. 3,254,023 and 3,267,023 to Miale et al.
U.S. Pat. No. 3,845,150 to Yan and Zahner teaches a heat balanced process for the aromatization of hydrocarbon streams by combining the exothermic aromatization of light olefins with the endothermic aromatization of saturated hydrocarbons in the presence of a medium-pore zeolite catalyst.
U.S. Pat. No. 4,431,519 to La Pierre et al. teaches a process for the hydrodewaxing of distillate in which an organic oxygenate reacts exothermically in the dewaxing reaction zone to supply the endothermic heat of reaction for the catalytic dewaxing process.
Aromatization of C.sub.3 -C.sub.4 paraffin-rich streams (commonly known as LPG), is highly endothermic. The aromatization reaction may be carried out in a fixed, moving or fluid catalyst bed. For example, the CYCLAR (tradename) process for LPG aromatization uses a plurality of moving-bed reaction zones together with continuous catalyst regeneration (CCR) to supply the required heat for the primary endothermic reaction. This commercial process scheme involving transporting hot catalyst pellets between the reaction and regeneration zones requires extensive capital investment.
The CYCLAR (tradename) process is described in the paper "CYCLAR: One Step Processing of LPG to Aromatics and Hydrogen," by R. F. Anderson, J. A. Johnson and J. R. Mowry presented at the AIChE Spring National Meeting, Houston, Tex., Mar. 24-28, 1985.
From the foregoing it can be seen that the combination of endothermic reactions with exothermic reactions for the purpose of heat balancing is desirable and can be particularly advantageous when both reactions act in concert to yield useful products. The combination of such reactions to provide a substantially heat balanced reaction zone would be still more beneficial if feedstreams having a relatively low economic value could be upgraded in such a heat balanced reaction zone to provide product streams having a substantially increased economic value.
The availability of liquified petroleum gas (LPG), specifically butane, is expected to increase in the near future. Butane is presently a valuable gasoline blending component which provides among other benefits, excellent winter cold-starting characteristics for automotive gasolines. Butane improves cold starting by readily volatilizing inside the engine cylinders. Unfortunately, butane's relatively high volatility raises the vapor pressure of the gasoline. Environmental concerns relating to evaporative gasoline losses to the atmosphere have prompted more stringent regulations requiring motor gasolines to be less volatile.
Rather than sell butane at lower valued LPG, it would be preferable to convert this stream to a high octane blending component having acceptable volatility (vapor pressure) characteristics. As mentioned above, it would be still more preferable to upgrade butane and other light C.sub.4 -paraffins while avoiding high capital and operating costs associated with strongly endothermic paraffin aromatization.